b 1. Field of the Invention
This invention relates to a turbo blower for a gas laser for forcibly circulating laser gas through the gas laser, and more particularly to a turbo blower which has prolonged service life of bearings used therein.
2. Description of the Related Art
Recent carbon dioxide lasers (CO.sub.2 laser oscillators) of a fast axial flow type are compact in size and capable of generating a high-quality laser beam with high power output. They are widely used in laser beam machining applications, such as cutting of metal and nonmetal materials, welding of metal materials, and so forth. Especially, some of them are rapidly developing as CNC laser beam machines associated with CNC (Computer Numerical Control) systems for use in machining fields requiring high-speed and high-accuracy cutting of complicated shapes.
In a carbon dioxide laser, about 20% of the applied electric energy is converted into a laser beam and the rest is consumed to heat the laser gas. On the other hand, theoretically, the gain of the laser oscillation is proportional to the minus (3/2) th power of the absolute temperature T, which makes it necessary to forcibly cool the laser gas to a temperature as low as possible to increase the oscillation efficiency. To this end, in a carbon dioxide laser of a fast axial flow type, a turbo blower is used for forcibly circulating the laser gas in the laser to cause the laser gas to pass through a cooling unit.
FIG. 6 shows the construction of a conventional turbo blower 100A for lasers. The turbo blower 100A illustrated in the figure has a turbo impeller 1A mechanically connected to a shaft 80 by a nut 7A. The shaft 80 has a rotor 3A rigidly mounted on the periphery thereof by shrink fit. A stator 4A is provided around the rotor 3A. The stator 4A is fixed to a housing 120, and constitutes a high-frequency motor 30A together with the rotor 3A. The turbo impeller 1A is rotated by the high-frequency motor 30A at a speed of several tens of thousands of rpm.
The shaft 80 is rotatably supported by a pair of rolling bearings 50 and 60 arranged on both sides of the high-frequency motor 30A. For lubrication of the rolling bearings 50 and 60, grease is used. Further, the shaft 80 has radiating fins 63 and 64 rigidly mounted thereon by shrink fit for cooling the bearings.
In the turbo blower 100A constructed as above, the laser gas is drawn therein in the direction of an arrow 9A in FIG. 6, and discharged in the direction of an arrow 10A, i.e. in a centrifugal direction.
The conventional turbo blower 100A rotates at a high speed of several tens of thousands of rpm, as described above, which causes a fairly large amount of heat to be generated in the motor 30A and the shaft 80. The heat is mainly ascribed to core loss and copper loss caused by magnetic flux of revolving magnetic field. On the other hand, the turbo blower 100A only contains a thin laser gas at about 0.1 atmospheric pressure. Therefore, cooling effects of natural convection cannot be expected, while effects of forced cooling by the radiating fins 63 and 64 are not sufficient.
Therefore, heat generated in the rotor 3A and the shaft 80 propagates to the rolling bearings 50 and 60 without being sufficiently dissipated. When the rolling bearings 50 and 60 are thus heated to a high temperature, grease charged therein is deteriorated, eventually causing seizure of the bearings, giving damage thereto.
Further, a rise in temperature of the rolling bearings 50 and 60 accelerates evaporation of grease, and a portion of the resulting grease vapor is mixed with the laser gas to contaminate optical components of the carbon dioxide laser. The contamination of the optical components brings about problems of decrease in power output of the laser and defective machining characteristics thereof resulting from a deformed laser beam generated thereby.
Further, since the radiating fins 63 and 64 are provided on the shaft 80 for cooling the bearings, the overall length of the shaft 80 is increased by an amount corresponding to the radiating fins 63 and 64. Therefore, the rotational speed of the turbo impeller 1A should clear both a first-order resonance point (critical speed) and a second-order resonance point (critical speed) before it reaches its maximum, and hence a damping structure has to be provided to suppress the vibration occurring at these resonance points. Further, in correcting the balance of rotating elements, such as the turbo impeller 1A and the like, a very large amount of labor is required for securing proper accuracy, which results in poor workability of the turbo blower in assembly and maintenance thereof.